Apparatuses of this type are used for the rapid and accurate determination of the air permeability of sheet-like structures of all types in the laboratory or directly during production or treatment of such sheet-like structures, for example textile webs for the production of balloon envelopes, parachutes, and geotextiles, and also wire meshes, etc. Since the air permeability is a measure of the density of a sheet-like structure, the determination of the air permeability can also be used directly for detecting structural changes in the inspected cloth web.
A known apparatus of this generic type includes a measuring head with a cylindrical measuring tube which opens through a surface of the measuring head in order to form a typically circular measuring orifice. The suction side of a pump is connected to the measuring tube. To determine air permeability, a test sample of the cloth web is pressed between the measuring orifice and a pressure ring coordinated with the size of the measuring orifice, and air is sucked by means of the pump through the measuring orifice, the clamped test sample and the pressure ring. The air permeability is the speed of the air flow under a given pressure drop across the test sample. The flow speed of the air sucked through the test sample is measured in a flowmeter, and is indicated as the air permeability of the test sample by an indicator instrument.
However, this known apparatus is designed for measurements on a stationary test sample. It is not suitable for determining the air permeability on a running web. This is because, for this, the measuring head and the pressure ring would have to be moved in synchronism with the running web, because satisfactory sealing around the circumference of the measuring orifice by means of a stationary pressure ring is impossible when a web is running. This would, however, lead to constructions involving a very high cost. On the other hand, a determination of the air permeability on a running web, for example in a treatment installation in which a web is guided continuously in a spread-out state under a regulated tension, would be highly desirable, since stopping of the installation for the purpose of measuring the air permeability could be avoided.